A piezoelectric resonator of this type with small dimensions has already been disclosed in the prior art document U.S. Pat. No. 7,626,318, which is incorporated herein by reference. The appended FIG. 1 is a reproduction of the FIG. 1 of this prior art document. Referring to this figure, one can see that the depicted tuning-fork resonator 1 is formed of a base 4 and two vibrating arms 2, 3 extending from the base. In a known manner, each arm carries metalized portions 5, 6 that form central electrodes on the main opposite surfaces and lateral electrodes along the sides of each arm. The central electrodes on one arm are connected to the lateral electrodes on the other arm and to one terminal of an oscillator circuit. The other central and lateral electrodes are connected in an identical manner to the other terminal of the oscillator circuit. The oscillator circuit is an electrical circuit arranged to subject the electrodes to an oscillating electric current that causes the arms to vibrate at a desired fundamental frequency.
Grooves 7, 8 are provided in each arm. The arrangement of the central electrodes inside the grooves, in the thickness of the arms, increases piezoelectric coupling. For a resonator having the same dimensions, the increase of the coupling causes a decrease in the equivalent resistance for a given resonator quality factor and consequently a decrease in the power consumption thereof. Conversely, for the same equivalent resistance, the presence of the grooves allows a reduction in the dimensions of the resonator. In substance, the use of grooves makes it possible to produce highly miniaturized resonators with the same given resonant frequency.
There is however a limit to miniaturization. Indeed, theory shows that, if the length of the vibrating arms is L and the width of the arms is W, the fundamental resonant frequency tends to behave like W/L2. It follows that, for a given resonant frequency, the shorter the length of an arm is, the narrower its width must be. A consequence is that, below a certain dimension, the vibrating arms must be made so narrow that the reduction in the volume of the elastically active piezoelectric material is associated with an unacceptable decrease in the quality factor. Notwithstanding, in order to attain a higher degree of miniaturization, the above-mentioned prior art document further teaches that the vibrating arms each end in an enlarged portion called a flipper 9, 10. The flippers have the effect of lowering the frequency for a given length, and thus allow having shorter vibrating arms for the same given frequency.
It is an object of the present invention to provide a piezoelectric resonator compatible with a higher degree of miniaturization than prior art resonators, without overly reducing the quality factor or increasing the equivalent resistance.